The other reason certified email won't solve the spam problem
The other problem with certified email as a way to solve the spam problem is a basic idea:
People don't pay for what they don't need.
So ask yourself: who actually needs to pay for a certification that their email isn't spam?
Significant sources of good email certainly don't need to pay; people already want email from them and will raise heck if stuff starts getting blocked. No one can afford to block GMail, for example.
There's some motivation for smaller sources of good email to pay up, but this only works if big ISPs and email providers do anything with the certification, which is unlikely. And even if it works, the smaller sources are going to feel that they're being held to ransom, since they have to pay for something that the big sources don't.
There's a big motivation for senders of unsolicited email to pay up, since people they're sending it to don't know to complain to their ISPs if it doesn't arrive. However, this is the sort of email that gets the most complaints and winds up being the least wanted overall.
(Some of it will be genuine spam from genuine spammers; some will just be newsletters or helpful notices or the like that people forgot about, no longer want, or didn't realize that they were signing up to.)
Or in short: the people who most need their email certified as good are the people who's email has the highest chance of being unwanted. This does not exactly create a strong motivation for people to accept the certification.